1. Field of the Invention
The invention relates to methods for direct injection of CO2 into the ocean for carbon sequestration. More particularly, it relates to the production of a negatively buoyant CO2 hydrate in the form of a consolidated CO2-hydrate/CO2-liquid/water stream that sinks upon release at intermediate ocean depths of about 1000 m.
2. Background Information
The concentration of carbon dioxide (CO2) in the atmosphere is steadily increasing as a result of both land use changes and the combustion of fossil fuels for energy production. Due to the enhanced greenhouse effect caused by increasing concentrations of CO2 and other greenhouse gases in the atmosphere (e.g., methane), it is predicted that greater amounts of heat will be retained within the atmosphere leading to a gradual increase in the surface temperature of the earth. Reducing the potential risks of human-induced global climate change will require that means be found to slow the rate of increase in atmospheric CO2 levels. One of the strategies is to capture and sequester CO2 by enhancing the natural capacity of the terrestrial biosphere and the oceans to take up and store carbon.
Direct injection of CO2 into the ocean has been proposed as a means for carbon sequestration because it offers a large storage capacity for carbon (Herzog 1998). Depending on the depth of injection as well as the subsequent interaction of CO2 with seawater, the residence time of CO2 in the ocean can be on the order of several hundred years, leading to significantly reduced rates of atmospheric CO2 increase as well as lower peak levels. The thermodynamic properties of CO2 and seawater, in combination with ambient pressure and emplacement methodology, will strongly influence the form and subsequent fate of CO2 that is injected into the ocean. For example, at depths less than xcx9c500 m, CO2 will be a gas and will therefore be more likely to partition back into the atmosphere within decades to centuries. At depths between xcx9c500 and xcx9c2600 m, the density of liquid CO2 is lower than that of seawater. At greater depths, liquid CO2 is denser than the surrounding seawater. Thus, CO2 injected at depths between 500 and 2600 m will be in liquid form and will tend to rise (i.e., be positively buoyant), while CO2 released at depths  greater than 2600 m will sink (i.e., be negatively buoyant).
Direct ocean CO2 injection will be considered successful if the following conditions are met: the residence time of CO2 released in the ocean is on the order of several centuries or more; negligible environmental impacts are associated with the release; the energy requirement for the ocean emplacement is small relative to that obtained from CO2 generation; and the process is cost-effective.
Several methods for direct CO2 injection have been suggested. These include: (1) injection at moderate depths of 1000-2000 m through a fixed or towed pipe resulting in a rising liquid CO2 droplet plume; (2) injections into ocean floor depressions at depths  greater than 2600 m forming a CO2 lake; (3) disposal as dry ice; and (4) shallow discharge as a dense solution of seawater with dissolved CO2 forming a dense sinking liquid plume. These and other methods are reviewed in the recent papers of Caulfield (1997) and Herzog (1998).
Because emplacement costs increase significantly with injection depth, the lowest cost is anticipated for the dense-plume approach (alternative 4), which requires injection depths between 500 and 1000 m. However, the low cost of implementation for this approach may be offset by the negative environmental impact on the marine ecosystem that would result from a highly concentrated CO2 composition and low pH in the vicinity of the sinking dense plume. Injection at depths  greater than 1000 m is therefore believed to have lesser environmental impacts and lower rates of release to the atmosphere. A high cost is associated with the CO2-lake disposal (alternative 2) because of the need for special pipelines that can withstand hydrostatic pressures at the required injection depth ( greater than 2600 m) where CO2 becomes denser than seawater. Dry ice (alternative 3) can be discharged at shallow depths, however, its production and handling cost can be very high.
When compared to the other disposal alternatives, droplet plume disposal at injection depths of 1000-2000 m (Alternative 1) appears to be the most favorable when factors such as development cost, difficulty and environmental impacts are considered. As CO2 is only slightly miscible with seawater, the CO2-seawater system is hydrodynamically unstable, and liquid CO2 discharged into seawater will break up into droplets due to interfacial instability. The droplets will rise because injection depths are shallower than the xcx9c2600-m required for CO2 to be negatively buoyant in seawater. To ensure that the rising CO2 droplets completely dissolve into the seawater before it reaches depths where CO2 becomes gaseous (xcx9c500 m), sufficient injection depth ( greater than 1500 m) is required.
The preceding review of current research shows that the positive buoyancy of CO2 droplets has a negative impact on the long-term environmental success of liquid CO2 injections at intermediate depths. In addition, although CO2 is in liquid state at depths  greater than 500 m, injections must be performed at depths greater than 1500 m to ensure that rising CO2 drops dissolve completely before reaching the critical 500-m depth threshold.
Our invention is a CO2 injection method based on the production of a new CO2 injection form, comprising of a consolidated CO2-liquid/CO2-hydrate/water paste-like stream, that sinks at shallower depths than other CO2 forms. To date, no studies discussing generation of a negatively buoyant CO2-liquid/CO2-hydrate/water consolidated stream for ocean sequestration have been reported. The result is the achievement of cost savings without the negative environmental impact of other shallow depth injection methods.
1. J. A. Caulfield, D. I. Auerbach, E. E. Adams and H. J. Herzog, xe2x80x9cNear Field Impacts of reduced pH from Ocean CO2 Disposalxe2x80x9d, Energy Convers. Mgmt. Vol. 38, pp. S343-348 (1997).
2. H. J. Herzog, xe2x80x9cOcean Sequestration of CO2xe2x80x94 An Overviewxe2x80x9d, Fourth International Conference on Greenhouse Gas Control Technologies, Interlaken, Switzerland, pp. 1-7, Aug. 30-Sep. 2, 1998.
3. J. J. Morgan, V. R. Blackwell, D. E. Johnson, D. F. Spencer and W. J. North, xe2x80x9cHydrate Formation from Gaseous CO2 and Waterxe2x80x9d, Environ. Sci. Technol. Vol. 33, pp. 1448-1452 (1999).
4. S. Hirai, Y. Tabe, G. Tanaka and K. Okazaki, xe2x80x9cAdvanced CO2 Ocean Dissolution Technology for Longer Term Sequestration with Minimum Biological Impactsxe2x80x9d, Greenhouse Gas Control Technologies, P. Riemer, B. Eliasson and A. Wokaun, editors, Elsevier Science, Ltd., pp. 317-322 (1999).
5. A. Yamasaki, M. Wakatsuki, H. Teng, Y. Yanagisawa and K. Yamada, xe2x80x9cA New Ocean Disposal Scenario for Anthropogenic CO2: CO2 Hydrate Formation in a Submerged Crystallizer and its Disposalxe2x80x9d, Energy Vol. 25, pp. 86-96 (2000).
6. T. J. Phelps, D. J. Peters, S. L. Marshall, O. R. West, L. Liang, J. G. Blencoe, V. Alexiades, G. K. Jacobs, M. T. Naney and J. L. Heck, Jr., xe2x80x9cA New Experimental Facility for Investigating the formation and Properties of Gas Hydrates under Simulated Seafloor Conditionsxe2x80x9d, Rev. Sci. Instrum. Vol. 72, No. 2, pp. 1514-1521 (2001).
It is a first object of the invention to provide a consolidated CO2-hydrate/CO2-liquid/water stream that sinks upon release at intermediate ocean depths of about 1000 m.
Another object of the invention is to reduce pressurization of CO2 liquid for ocean injection by providing a negatively buoyant CO2 stream for injection at shallower depths.
Another object of the invention is to provide a CO2 injection form having a longer residence time in the ocean.
A further object of the invention is to dissolve CO2 slowly, imposing minimal environmental impact.
A still further object of the invention is to provide efficient and economical CO2 disposal in the ocean.
Yet another object of the invention is to provide a CO2 disposal method that is compatible with current pipeline delivery systems.
In a first embodiment, the invention is a method for continuous production of a hydrate-containing stream that comprises the steps of delivering a fluid containing hydrate-forming species to a pressurized, temperature controlled, continuous-flow reactor; and mixing the fluid containing hydrate-forming species with water until a consolidated hydrate-fluid-water stream is formed.
In another embodiment, the invention is a method for sequestering CO2 in the ocean that comprises the steps of pumping liquid CO2 into a discharge pipe located at a predetermined ocean depth; pumping seawater from the predetermined ocean depth into the discharge pipe; sufficiently mixing the liquid CO2 with the seawater from the predetermined ocean depth for a sufficient amount of time until a paste-like consolidated CO2-hydrate/CO2-liquid/water stream is formed; and discharging the paste-like consolidated CO2-hydrate/CO2-liquid/water stream into the ocean.